1. Field of the Invention
The present invention relates to bearings for use in vacuum chambers. More particularly, the invention relates to the application of such a device in lithographic projection apparatuses.
2. Discussion of Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatuses are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In a lithographic apparatus, the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices and hence higher operating speeds, it is desirable to be able to image smaller features. Whilst most current lithographic projection apparatuses employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include laser plasma sources or synchrotron radiation from electron storage rings. An outline design of a lithographic projection apparatus using synchrotron radiation is described in xe2x80x9cSynchrotron radiation sources and condensers for projection x-ray lithographyxe2x80x9d, J B Murphy et al, Applied Optics Vol. 32 No. 24 pp 6920-6929 (1993).
Other proposed radiation types include electron beams and ion beams. These types of beam share with EUV the requirement that the beam path, including the mask, substrate and optical components, be kept in a high vacuum. This is to prevent absorption and/or scattering of the beam, whereby a total pressure of less than about 10xe2x88x926 millibar is typically necessary for such charged particle beams. Wafers can be contaminated, and optical elements for EUV radiation can be spoiled, by the deposition of carbon layers on their surface, which imposes the additional requirement that hydrocarbon partial pressures should generally be kept below 10xe2x88x928 or 10xe2x88x929 millibar. Otherwise, for apparatuses using EUV radiation, the total vacuum need pressure only be 10xe2x88x923 or 10xe2x88x924 mbar, which would typically be considered a rough vacuum.
Further information with regard to the use of electron beams in lithography can be gleaned, for example, from U.S. Pat. No. 5,079,122 and U.S. Pat. No. 5,260,151, as well as from EP-A-0 965 888.
Working in such a high vacuum imposes quite onerous conditions on the components that must be put into the vacuum and on the vacuum chamber seals, especially those around any part of the apparatus where a motion must be fed-through to components inside the chamber from the exterior. For components inside the chamber, materials that minimise or eliminate contaminant and total outgassing, i.e. both outgassing from the materials themselves and from gases adsorbed on their surfaces, should be used. It would be very desirable to be able to reduce or circumvent such restrictions.
Bearings in vacuum are a particular problem. Most lubricants are unsuitable for use in high vacuum conditions, particularly when low hydrocarbon partial pressures are required. Unlubricated bearings are known, but are subject to wear and cannot meet the speed of operation and lifetime requirements of lithography apparatuses. It is also difficult with conventional bearings to reduce the bearing gap below about 30 xcexcm. Such a gap around a motion feed-through into the vacuum chamber would present an unacceptable leak.
It is an object of the present invention to provide an improved bearing that can be used in a vacuum chamber of a lithographic projection apparatus, e.g. to support a slidable plate sealing an aperture in the vacuum chamber, a member passing through an aperture in the vacuum chamber or a moveable object within the vacuum chamber, and can operate at high speed for a very great number of cycles.
According to the present invention, these and other objects are achieved in a lithographic projection apparatus that has:
a radiation system for supplying a projection beam of radiation;
a first object table provided with a mask holder for holding a mask;
a second object table provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate, the lithographic projection apparatus has
a vacuum chamber having a wall enclosing at least one of the first and second object tables, the vacuum chamber wall having an aperture therein;
a moveable sealing member for sealing the aperture;
a bearing for bearing the sealing member and maintaining a gap, between the sealing member and the vacuum chamber wall, the bearing including
a gas bearing for providing pressurised gas into the gap thereby to generate forces tending to hold the sealing member away from said vacuum chamber wall; and
an evacuation component spaced apart from the gas bearing for removing the gas from the gap.
Another aspect of the invention provides a lithographic projection apparatus that is:
a radiation system for supplying a projection beam of radiation;
a first object table provided with a mask holder for holding a mask;
a second object table provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate. The lithographic projection apparatus also has
a vacuum chamber having a wall enclosing at least one of the first and second object tables, the one object table being movable;
a bearing for displaceably bearing the one object table against a bearing surface within the vacuum chamber and maintaining a gap therebetween, the bearing including
a gas bearing for providing pressurised gas into the gap thereby to generate forces tending to separate the borne and bearing members; and
an evacuation component spaced apart from the gas bearing for removing gas from the gap, the evacuating component being provided to surround the gas bearing.
Current lithography apparatuses are designed for use in clean room environments and therefore some steps have conventionally been taken to reduce possible sources of contamination of wafers that are processed by the apparatus. However, conventional designs of wafer, mask and transfer stages are very complicated and employ large numbers of components for sensor and drive arrangements. Such stages also need to be provided with large numbers of signal and control cables and other utilities. The present invention avoids the difficult and detailed task of making such large numbers of components vacuum-compatible, or replacing them with vacuum-compatible equivalents, by adopting the principle of locating as many components and functions as possible outside the vacuum chamber. The present invention thus avoids the need to vacuum-proof many or most of the numerous components, by providing appropriate mechanical feed-throughs with innovative sealing arrangements. Likewise, the present invention avoids difficulties in reducing vibrations inevitable in vacuum apparatuses, particularly where powerful pumps are provided, by isolating as far as possible vibration sensitive components from the vacuum chamber wall.
The gas bearing of the invention maintains the desired separation between the borne and bearing members (e.g. the sealing member and the vacuum chamber wall or the object table and bearing surface in the vacuum chamber) , whilst the evacuation component prevents the gas forming the gas bearing being an unacceptable leak into the vacuum chamber. The vacuum bearing of the invention has many applications, for example supporting a sliding seal plate, supporting a rod of a motion feed-through into the vacuum chamber, supporting, an object or object table that must move on the vacuum chamber floor, or allowing a support pillar to be isolated from the vacuum chamber wall.
The evacuation component may comprise an elongate groove located in one surface defining the gap and connected by spaced-apart vacuum conduits to a vacuum pump. Alternatively, multiple parallel grooves may be provided and connected to separate vacuum pumps, with those grooves nearer the vacuum chamber drawing a deeper vacuum.
According to a further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus that has:
a radiation system for supplying a projection beam of radiation;
a first object table provided with a mask holder for holding a mask;
a second object table provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate. The lithographic projection apparatus also has
a vacuum chamber having a wall enclosing at least one of the first and second object tables, the vacuum chamber wall having an aperture therein;
a moveable sealing member for sealing the aperture;
a bearing for bearing the sealing member and maintaining a gap between the sealing member and the vacuum chamber wall, the bearing comprising:
a gas bearing for providing pressurised gas into the gap thereby to generate forces tending to hold the sealing member away from the vacuum chamber wall; and
evacuation means spaced apart from the gas bearing for removing the gas from the gap; the method includes
mounting a mask on said first object table;
mounting a substrate on the second object table; and
exposing the substrate to an image of the mask.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping) metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 007-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
The present invention and its attendant advantages will be described below with reference to exemplary embodiments and the accompanying schematic drawings, in which:
FIG. 1 depicts a lithographic projection apparatus according to a first embodiment of the invention;
FIG. 2 is a cross-sectional view of a differential gas bearing according to a second embodiment of the invention;
FIG. 3 is a plan view of a variation of the differential gas bearing of FIG. 2;
FIG. 3A is an enlarged cross-section of part of the differential gas bearing of FIG. 3;
FIG. 4 is a cross-sectional view of a wafer stage of a lithographic apparatus according to a third embodiment of the present invention;
FIG. 5 is a cross-section of an isolation mount according to a fourth embodiment of the invention; and
FIG. 6 is a cross-section of a wafer stage according to a fifth embodiment of the present invention.
In the various drawings, like parts are indicated by like references.